1. Field of the Invention
The present invention relates to an ion concentration analyzer of the flow cell type which uses an ion selecting electrode to determine ion concentration in blood and urine. 2. Description of the Related Art
The ion concentration analyzer usually uses a rolling pump or a syringe pump as means for introducing sample liquid into the flow cell. It is more preferable to use the syringe pump in the case where the amount of sample liquid introduced is not changed by aging and an assay of high accuracy is required.
The conventional syringe, usually used for this syringe pump, has the structure shown in FIG. 3. A sliding block made of elastic resinous or rubber insulating material suited for sealing such as polytetrafluoroethylene (hereinafter referred to as PTFE) is fixed to the distal end of a metal piston rod 3, which is slidably inserted into a glass or plastic cylinder 1. This syringe thus arranged is often used in automatic analyzers, because it can introduce a predetermined amount of liquid with high accuracy and efficiency, and also because it is highly durable.
When the sliding block 2 is worn out, however, the liquid leaks from the syringe at the portion worn out, and will contact the metal piston rod 3. Consequently, voltage noise is generated while the syringe is operated.
The ion concentration analyzer, which has an ion selecting electrode to measure a low voltage, must be provided with means for preventing any influence of the voltage noise on the ion selecting voltage.
As disclosed in Published Unexamined Japanese Patent Application No. 60-143756, one of the conventional methods of avoiding the voltage noise is to connect an a ground to a portion adjacent to an electrode for measuring an electric voltage so as to discharge the voltage noise out of the ion concentration analyzer.
The ion concentration analyzer of a flow cell type, in which this noise-preventing measure is taken, is shown in FIG. 3. The ion concentration analyzer comprises a flow cell 6, a sucking nozzle 5, and a syringe 12. The flow cell 6 has openings through which sample liquid will be fed and discharged. The a sucking nozzle 5 is connected to one of the openings of the flow cell 6. The syringe 12 is connected to another opening of the flow cell 6 by a metal pipe 9 and a check valve 11a.
More specifically, an ion selecting electrode 7 and a reference electrode 8 are attached, at one end, to the flow cell 6 and, at the other end, to an arithmetic unit 15. The arithmetic unit 15 calculates the ion concentration of sample liquid. Further, the sample sucking nozzle 5 is connected to a nozzle drive system 4 and can be appropriately lowered to a sample 13 transferred below the nozzle 5 to suck the sample 13.
As has been described with reference to FIG. 3, the syringe 12 comprises the glass cylinder 1 and a piston slidably inserted in the cylinder 1. The piston has the sliding block 2 which is made of elastic insulating matter such as polytetrafluoroethylene and secured to the distal end of the metal piston rod 3. The piston rod 3 has a flange at the proximal end. The flange is connected to a syringe drive mechanism 16. Hence, the mechanism 16 can drives the piston rod 3 back and forth in the glass cylinder 1.
A pipe line 17 is connected, at one end, to the syringe 12 and to an outlet port, at the other end. A check valve 11b is provided on the pipe line 17.
The metal pipe 9 arranged on the liquid flow path is connected to a ground, thereby grounding the sample liquid flowing through the pipe line connecting the flow cell 6 to the syringe 12. Hence, the pipe 9 prevents voltage noise from reaching the flow cell 6.
However, the method of grounding the sample liquid, described above, cannot be effectively used in a conventional ion concentration analyzer of the type, wherein the sample liquid is grounded via another ground at a point adjacent to another noise-generating source including a power supply source, nor can the method be used successfully in a conventional ion concentration analyzer of the type, wherein a plurality of cells is connected to the syringe by pipes. This is because multi-point grounds are provided and because current flows between these grounds when grounding is made between each of the flow cells and the syringe, thus causing it to detect an electric voltage including the noise. Moreover, the provision of a ground per se may expose an operator to a danger of an electric shock.